Silicone Brake Fluid in Miata?

All it takes is to whisper "Silicone Brake Fluid" on the brit car boards,and a flame war will erupt. Hopefully this wont be the case today.

That said, I have always run DOT5 in the Brit cars that I have owned and not had any issues. Conversely, any Brit car that I have not used it in, or come across with conventional fluid, has paint damage at the firewall due to leaking DOT3 fluid.

So, since I hate seeing expensive paint destroyed, (currently refinishing the brake booster for just this reason) my intentions are to run DOT5 in the Molvo. That is unless there is some sound reason not to.

If your Miata has ABS, Do NOT use DOT 5 silicon fluid. It will render your ABS useless as the minuscule air bubbles trapped in the DOT 5 fluid expand and collapse under extreme braking. No ABS, you should be fine unless you track the Miata (see reason above).

As an aside, DOT 5 was all the rage in the late 70's and early 80's for the C2/C3 Corvettes as the stock calipers were cast iron and DOT 3 fluid was (and still is) very hydroscopic. Pitted piston bores that failed to seal were a real problem unless you changed out fluid yearly or at every other club racing track event. DOT 5 solved the pitting problems, but gave a spongy, pulsating brake at track and auto-x events (brake feedback was very similar to ABS). The result of all this is the prevailing "flame wars" you mentioned in your post.

Good Luck with your Miata. It's a fun car to drive!

All kinds of terrible things can happen if you put DOT5 into a system that has any components not meant for it. Even if you build an ideal DOT5-based brake system, it will be junk compared to any average modern-ish brake system because DOT5 fluid in peak condition acts almost exactly like old DOT3/4/5.1 fluid that desperately needs changing, except that the boiling point is higher. The DOT5-based system will probably also leak earlier since it doesn't absorb water out of the braking system (again like old DOT3/4/5.1 that needs changing), allowing rust to progress faster...but as a consolation prize, when the fluid drips out it won't harm the paint.

I've only suffered one rust spot from leaking brake fluid on any of my cars (and I've fixed it). If you rinse spilled brake fluid away quickly with water it won't do any harm. It's not all that bad.

If you want to build an exotic braking system for an alternative fluid, you might want to consider mineral oil, as used on some motorcycles and MTBs:

http://www.epicbleedsolutions.com/blog/dot-brake-fluid-vs-mineral-oil/

The only downsides of a mineral oil braking system vs. a traditional DOT-type braking system are the cost of the fluid and extreme vulnerability to water ingress - the fluid doesn't absorb water so it will only enter the system via splashing or condensation, but if it does, it forms boilable pockets that will cause abrupt and severe hydraulic system overheating.

Some observations from 25 years of MGB and Healy use of DOT5

Myth: You must start with a fresh braking system since the two will not be compatible in any amount. Always just pumped the old glycol out and bled the brakes

DOT5 wont absorb water: Water does enter the system cause water is in air. It wont mix with the DOT5 so it creates water "bubbles" and localized rust. ...flush the stuff same as if it were glycol. This might be its biggest failing as people who use it think they never have to change the stuff.

DOT5 wont work on the track for lapping: Done it and under the same caveat of you gotta change the fluid on occasion, seems to work fine. Would not use for racing cause that's another game.

British rubber is made from trees and will swell and cause brake failure. Maybe true, but is it still relevant?

You gonna have a soft pedal with DOT5. Not that I ever noticed.

Harder to bleed. Not that I have noticed, but then I don't go all 50 shades of grey on the brake pedal when bleeding either.

I wont be running ABS and so far that seems to be the most compelling reason not to use it in newer cars.

I change all my clutch hydraulics to DoT5 whenever I have to service them. Works great.

My Harleys have DoT5 in the brakes. Works great. I didn't touch my rear brake system for about 15 years. Not even check the fluid level. Just left it alone. Finally, I was starting to have a soft pedal. So, assuming that the whole caliper and MC would be junk, I bought rebuild kits. When I tore the system apart, the caliper, with >70K miles on it, looked practically new inside. Light scuff marks from the piston. Same for the MC. I put the kits in as I had bought them and put it all back together. Couldn't get it to bleed. One of the copper crush washers joining the lines had a very small leak.

I have been running DoT4 in the cars.

Oh, wanna buy DoT5 fluid cheap? Get the Mil Spec stuff on eBay. You can get it for like $20-30/gallon. It was used in the M1 Abrams brakes because the Army doesn't like systems that go bad either.